1. Field of the Invention
This invention relates to the art of electrophotography and more particularly, to photosensitive materials which have a double-layer structure of a charge generation layer and a charge transport layer and which are particularly suitable for use in an electrophotographic process including charging, exposing and developing operations to form images.
2. Description of the Prior Art
Organic photoconductive conductors using organic photoconductive materials have a number of advantages over inorganic photoconductive conductors, including the ease in preparation of a variety of materials exhibiting high sensitivity at different wavelengths depending on the molecular design, little or no ecological problem, good productivity and economy, and inexpensiveness. Accordingly, extensive studies have been hitherto made on such organic conductors. Some organic conductors are in use and, at present, are being mainly employed as photosensitive materials for electrophotography.
Known organic photoconductive conductors are usually arranged to have a double-layer structure which includes a charge generation layer capable of absorbing light to generate carriers and a charge transport layer wherein the generated carriers are transported. Many attempts have been made to higher sensitivity. Known organic conductive materials used to form the charge generation agent include various perylene compounds, various phthalocyanine compounds, thiapyrylium compounds, anthanthrone compounds, squalilium compounds, bisazo compounds, trisazo pigments, azulenium compounds and the like.
On the other hand, the materials developed to form the charge transport layer include various hydrazone compounds, oxazole compounds, triphenylmethane compounds, arylamine compounds and the like.
The charge generation and transport agents are, respectively, coated along with polymer binders by relatively simple coating techniques on supports such as drums, belts and the like. Examples of the binders used for this purpose include polyester resins, polycarbonate resins, acrylic resins, acryl-styrene resins and the like. In order to attain high sensitivity by the use of the double-layer structure, it is general that the charge generation layer is applied in a thickness of 0.1 to 1 micrometer and the charge transport layer is applied in a thickness of 10 to 20 micrometers. From the standpoint of the physical strength and the printing resistance, the charge generation layer is formed directly on the substrate and the charge transport layer is formed as a surface layer. In this arrangement, charge transport agents which are now in use are those which act by movement of positive holes. Thus, the known photosensitive materials are eventually of the negative charge type.
The negative charge systems, however, have involved the problem that images are apt to suffer the influence of a support surface. For instance, if an aluminium drum is used, impurities and especially, inorganic metal impurities inevitably contained in the aluminium and irregularities on the surface influence the image quality. More particularly, such impurities or irregularities reflect on image defects such as white and/or black spot defects on images. This becomes more pronounced under high temperature and high humidity conditions. Thus, satisfactory image characteristics cannot be obtained.
Such influences may be mitigated by use of highly pure drum materials, by mirror finish of the drum or by application of an undercoating on the drum, with an increase in production costs. Of these, the undercoating technique is considered to be most suitable. However, a difficulty is involved in an increase of residual potential.
In order to solve the above problems, there is known a positive charge system wherein the charge generation layer and the charge transport layer are superposed in a reverse order as of the case of a negative charge system. However, this positive charge system should be very thin in the charge generation layer as set forth before. This leads to poor mechanical strength and a poor printing resistance. Thus, the known positive charge system has a little utility in practical applications. If a protective layer is formed on the charge generation layer, the printing resistance may be improved but a residual potential will be increased with a lowering of environment resistance.